The invention relates to an operating method for a fuel cell system with a radiator structure flowed through by ambient air and a fuel cell stack, at least part of the outgoing air flow of which can be guided onto the radiator structure in such a way that, at the radiator structure, the guided outgoing air flow causes an increase in the mass flow of the ambient air through the radiator structure according to the jet pump principle.
A corresponding outgoing air guiding of a fuel cell stack is described in German patent applications having official file reference numbers 102012211421.1 (U.S. application Ser. No. 14/587,071) and 102013214602.7 (U.S. application Ser. No. 15/001,402). Such an outgoing air routing utilizes a part of the energy contained in the fuel cell outgoing air flow for delivering ambient air through a so-called radiator structure. The radiator structure is provided, in particular, for cooling the fuel cells of the fuel cell stack in the fuel cell system.
It is known, furthermore, that part of the energy which is contained in particular in the form of positive pressure in the outgoing air flow of a fuel cell or of a fuel cell stack can be recovered in a turbine or another so-called expander, wherein for this purpose reference is merely exemplarily made to DE 10 2011 122 306 A1. Such an expander or the like is described as a gas expansion machine here, seeing that therein the outgoing air flow exiting the fuel cell stack with a certain pressure is subjected in particular to a pressure reduction or said certain pressure is utilized for driving the turbine or similar device (gas expansion machine).
The aim here is to show how the energy contained in the outgoing air flow can be utilized even better in as favorable as possible a manner.
The solution to this object consists in an operating method for a fuel cell system with a radiator structure flowed through by ambient air and a fuel cell stack, at least part of the outgoing air flow of which can be guided onto the radiator structure in such a way that, at the radiator structure, the guided outgoing air flow causes an increase in the mass flow of the ambient air through the radiator structure according to the jet pump principle. At least part of the outgoing air flow of the fuel cell stack can be guided through a gas expansion machine and the division of the energy, which is contained in the outgoing air flow and can be recovered in the gas expansion machine and/or at the radiator structure according to the jet pump principle, is changed by an electronic control unit in a manner that is adapted to boundary conditions. Advantageous further developments are described herein.
According to the invention, different possibilities for recovering the energy contained in the outgoing air flow of a fuel cell are provided and a targeted division of the energy recovery over these different possibilities takes place namely (as generally known) over a gas expansion machine on the one hand and over the possibility of increasingly delivering ambient air through the mentioned radiator structure by way of the jet pump principle on the other hand. Before boundary conditions or aspects under which the division of the energy recovery specifically takes place are explained in more detail, it is initially explained how this division, as such, can be carried out in the first place.
Accordingly, the outgoing air flow of the fuel cell stack can be initially guided completely through the gas expansion machine and subsequently, as described in the patent applications mentioned at the outset, guided up to (or into) a radiator structure in order to bring about an increase of the mass flow of the ambient air through said radiator structure according to the jet pump principle. The guiding onto the radiator structure is thus located downstream of the gas expansion machine. The amount of energy that is now first removed in the gas expansion machine (and is thus no longer available for the utilization of the jet pump principle) can be determined through targeted adjusting of the flow conditions prevailing in the gas expansion machine, i.e. the pressure drop in the gas expansion machine is specifically adjusted for example by way of adjustable guide blades in a turbine (or the like). Alternatively, the gas expansion machine can be supplied with only a (first) part of the fuel cell outgoing air flow while the other (second) part of the fuel cell outgoing air flow is guided directly to the radiator structure in order to create (preferentially, again united beforehand with the air flow exiting the gas expansion machine) according to the jet pump principle the pressure drop which also delivers ambient air through the radiator structure. Obviously, a combination of these two stated alternatives is also possible.
As far as the boundary conditions are now concerned, by way of which an electronic control unit performs the division, according to the invention, of the energy recovery from the fuel cell outgoing air flow over the gas expansion machine on the one hand and the radiator structure on the other hand (here and in the following merely the term “radiator structure” is used for the sake of simplicity for guiding the fuel cell outgoing air flow onto the radiator structure in such a way that the same according to the jet pump principle brings about an increase of the mass flow of the ambient air through the radiator structure), the electronic control unit, measured directly or indirectly, takes into account in particular the temperature of the medium to be cooled in the radiator structure (=“coolant”) and thus practically the radiator output requirement of the fuel cell system. Here, in addition to or instead of the currently measured temperature (either of the medium to be cooled or of the radiator structure itself), a temperature that is estimated by calculation using suitable data and expected in the foreseeable future can also be taken into account. Directly or indirectly, the current electrical output of the fuel cell stack and/or the electrical current to be demanded in a foreseeable period of time can also be taken into account since the same does not only permit drawing conclusions regarding the current temperature conditions but also temperature conditions to be expected in a foreseeable period of time.
In the case of a fuel cell system according to the invention installed in a motor vehicle, the previously mentioned boundary conditions of an electronic system control unit are usually known. It is proposed to furthermore take into account the ambient temperature and/or the traveling speed of the vehicle equipped with the fuel cell system for the division. Obviously, the current pressure drop in the fuel cell outgoing air flow at the gas expansion machine can or should also be directly or indirectly included, preferentially in the form of a suitable proxy, in the calculations performed in the electronic control unit.
As already mentioned, the electronic control unit controls or regulates, as part of carrying out the operating method according to the invention, primarily the pressure drop of the fuel cell outgoing air flow at the gas expansion machine in a motor vehicle with a liquid-cooled fuel cell stack with the help, inter alia, of (i) the temperature of the cooling circuit of the fuel cell system, (ii) the traveling speed of the motor vehicle, (iii) the demanded output of the vehicle or of the fuel cell system, and (iv) the current pressure drop at the gas expansion machine. In particular with respect to the demanded output of the motor vehicle, a forward-looking calculation, for example with the help of GPS data, for example for an imminent uphill drive, can be conducted here. When subsequently an increase of the coolant temperature or of the expected coolant temperature over a predetermined limit value is present, the energy recovery in the gas expansion machine that is practically carried out up to that time is reduced, which results in a lowering of the system efficiency but simultaneously brings about a higher cooling output. Thus, a higher output of the fuel cell stack can be achieved inter alia at high ambient temperatures, in the case of high output requirements and at low traveling speed of the vehicle (for example when traveling uphill). When, by contrast, the cooling output requirement of the fuel cell system is low and no quasi additional cooling output worth mentioning is thus needed, the maximum energy recovery preferentially takes place from the fuel cell outgoing air flow in the gas expansion machine, wherein the latter applies generally—and thus not only for a fuel cell system installed in a motor vehicle.
Coming back to the use in a motor vehicle, it can thus be calculated in advance from the current vehicle speed and the output requirement from route data available in typical navigation systems or assumptions regarding the further traveling operation how the cooling requirement in general or in the case of a liquid-cooled fuel cell system the coolant temperature will develop in the future. When a temperature above a set maximum temperature is expected, the pressure drop at the gas expansion machine is already prophylactically reduced and a greater component of the energy recovery from the fuel cell outgoing air flow is supplied to the radiator structure as described above, i.e. the outgoing air flow is guided onto the radiator structure in such a way that the same at the radiator structure brings about an increase of the mass flow of ambient air through the radiator structure according to the jet pump principle.
When, by contrast, the vehicle is braked, a lower cooling requirement is to be expected and a larger component of the energy contained in the fuel cell outgoing air flow, preferentially in the form of a larger component of the fuel cell outgoing air flow itself, is supplied to the gas expansion machine in order to thereby increase the overall efficiency of the fuel cell system. However, the preconditioning of the cooling system of the fuel cell system or of the coolant of the (liquid-cooled) fuel cell stack for example for an imminent acceleration process of the vehicle should have priority.
It can be provided, furthermore, that in terms of an optimization of the system efficiency, the output requirement of a fan which, as is quite usual, delivers ambient air through the radiator structure is taken into account in the calculations of the electronic control unit and suitably adjusted, i.e. adjusted according to the boundary conditions and the outgoing air quantity currently guided onto the radiator structure according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.